In Release 10 and prior releases of Long Term Evolution (LTE) wireless communication system standards, the physical downlink control channels (PDCCH) carry all of the downlink (DL) and uplink (UL) scheduling information to inform individual wireless devices where to find information in the time and frequency resources utilized for transmissions between the wireless devices and a base station, such as an eNodeB (eNB). As used herein, DL refers to communications from the base station to the wireless device and UL refers to communications from the wireless device to the base station.
Such a typical wireless communication system 10 is shown schematically in FIG. 1. A network node 12, which can be a base station, is in communication with a backhaul network 14. The backhaul network 14 may include the Internet and/or the public switched telephone network (PSTN). The network node 12 communicates with a plurality of wireless devices 16a, 16b and 16c, referred to collectively herein as wireless devices 16. Although only one network node 12 is shown, an actual wireless communication system 10 has many network nodes, e.g., many base stations. Also, there will typically be more than three wireless devices 16.
In communication systems based on LTE standards, downlink communications between the network node 12 and the wireless devices 16 are based on a basic radio frame an example of which is illustrated in FIG. 2. A basic radio frame generally includes twenty slots which are paired together to form subframes, i.e., two slots for a subframe. In the more general sense, slots are combined to form subframes and subframes are combined to form frames. With additional reference to FIG. 3, each slot of the basic radio frame generally includes multiple resource elements (REs) which can be illustrated as a resource grid including multiple frequency carriers and multiple orthogonal frequency division multiplexing (OFDM) symbols. In the resource grid, one RE denotes a single OFDM symbol transmitted over a single frequency carrier. As illustrated in FIG. 3, OFDM symbols and frequency carriers can be grouped as physical resource blocks (PRBs). An LTE PRB generally includes 7 OFDM symbols over 12 frequency carriers for a total of 84 REs per PRB. However, these quantities can vary.
When two slots are combined into a subframe, as shown in FIG. 2, their combined resource elements are further divided into a control region which generally occupies the first 3 OFDM symbols (the first 4 OFDM symbols when the available bandwidth is 1.4 MHz) over the available bandwidth, i.e. over all the available frequency carriers, and a data region which occupy the remaining OFDM symbols, also over the available bandwidth. In FIG. 4, which illustrates an exemplary subframe, the shaded region is the control region while the non-shaded region is the data region.
The network node 12 generates and transmits a PDCCH which informs each wireless device 16 whether and when data is to be transmitted to the wireless device 16 or received from the wireless device 16. According to the aforementioned communication standards, a wireless device 16 must decode the PDCCH successfully in order to receive and send data. The PDCCHs are located within the control region of a subframe which, as indicated above, usually occupies the first 3 OFDM symbols at the beginning of each transmitted subframe.
The capacity of the PDCCH is a limiting factor in systems where there are a large number of wireless devices using low rate services such as voice over Internet protocol (VoIP). Indeed, due to the limited size of the control region, the number of PDCCHs than can be transmitted in any given subframe is limited. To alleviate the limitations of the control region, Release 11 of the LTE communication standards introduced an enhanced PDCCH (ePDCCH) which is transmitted on radio resources located within the data region of a subframe in which data are transmitted over physical downlink shared channels (PDSCH). Contrary to PDCCHs which are transmitted on radio resources distributed over the whole system bandwidth, ePDCCHs are transmitted on radio resources located on specific frequencies, or carriers, that can be allocated dynamically. In that regard, ePDCCHs employ frequency division multiplexing (FDM).
According to LTE communication standards, the available bandwidth of a communication system can be partitioned into subbands, where each subband comprises a number of PRBs which depends on the available bandwidth. Table 1 below sets forth the number of PRBs per subband as a function of the number of PRBs supported by the system bandwidth
TABLE 1System BandwidthSubband SizeBW(k)6-7NA 8-10411-26427-636 64-1108
Hence, according to Table 1, for a communication system with a system bandwidth supporting 16 PRBs, each subband will comprise 4 PRBs for a total of 4 subbands.
A wireless device can be provisioned with one or two sets of ePDCCH PRBs for ePDCCH monitoring. The ePDCCH PRB sets can consists of two, four or eight PRBs and the two sets maybe of different sizes. The location of these sets of PRBs and the configuration of these sets among the subbands can be provisioned as best suits the wireless device 16 and the network node 12, e.g., the eNodeB. In that sense, subband feedback is provided by the wireless device 16 to the network node 12 to indicate which sections of the frequency spectrum are preferred by the wireless device 16. Subband feedback reports are sent often as the channel qualities experienced by the wireless device 16 can change quickly. However, as a network node 12 may service several wireless devices 16 at any given time, trying to act on every subband feedback reports sent by every wireless device 16 would add a significant processing burden on the network node 12.